The present invention relates generally to sail boat design, and more particularly, pertains to watercraft that employ hydrofoils for support and an airfoil for wind powered propulsion.
Hydrodynamic drag comprises by far and away the greatest component of resistance to a boat's forward progress. It has long been recognized that such drag forces can be reduced by optimizing a hull's shape to reduce frontal or wetted area or by modifying a hull's shape to induce a planing attitude at speed which further reduces both frontal and wetted area. A much greater reduction in hydrodynamic drag can be achieved by raising the entire hull clear of the water by force generated by the flow of water across relatively small hydrofoil shaped surfaces. A hydrofoil of sufficient buoyancy can additionally support a boat's weight under static conditions thereby obviating the need for a conventional hull altogether. The concept of a buoyant hydrofoil is disclosed in U.S. Pat. No. 3,094,961 to Bernard Smith.
An additional shortcoming of "conventional" sailboat design is inherent in the behavior of a "conventional" sail. A properly shaped (filled) sail can, in fact, generate more propulsive force by acting as an airfoil with air flowing across its surface to produce "lift" than it can generate by simply catching the wind. The proper "filling" of a sail can, however, only be achieved by presenting the plane of the sail area at a significant angle to the apparent direction of the wind. Higher boat speed, therefore, requires the boat to be sailed further and further off the wind thereby reducing the boat's upwind performance. A solution to this dilemma has been the use of a rigid airfoil. Such a rigid structure will maintain a more optimized airfoil shape regardless of the wind direction and thereby generates higher propulsive forces. An airfoil shape symmetric with respect to the perpendicular bisector of the chord will generate "lift" when air flows in either direction across the foil's surfaces.
While airfoils and hydrofoils have in the past been combined in various high-performance sailing vessel designs, the mere combination of such features does not guarantee an efficient if indeed sailable watercraft. An additional obstacle that has, in fact, always impeded the fitting of large surface area sails to small watercraft, has been the requirement of stability. It is a typical watercraft's inherent instability for which counter measures must be taken to prevent the boat from overturning. In addition to limiting the sail area-to-weight ratio of a particular design, this instability problem has been addressed by the placement of ballast below the waterline, such as by the affixation of a weighted keel, or by shifting ballast from side to side as the boat heels. Another approach involves the use of multiple hulls. This has the effect of spreading out the forces to provide a more stable base. Stability of such structures, is however not unlimited, as sufficient sail area and/or wind velocity will eventually have the effect of overturning the vessel.
In order to construct a more stable craft, all of the force vectors that are involved must be considered, and with the ability to balance all of those force vectors that could result in a turning moment with equal and opposite forces, an "ultrastable" watercraft results that is not prone to turnover regardless of how high a wind velocity it is subjected to. Ultrastable water craft employing airfoils and hydrofoils have in fact been proposed as in the work of Bernard Smith entitled, "The Forty Knot Sailboat". While the various disclosed sailboats do offer high-speed potential and ultrastability, they suffer from shortcomings in regard to their maneuverability. For example, Smith's design requires a reversal in the direction of travel during downwind tacking maneuvers. In addition, the placement, configuration and the restricted freedom of movement of the control surfaces preclude any direct downwind movement (or running) and actually requires downwind tacking at significant angles to the direction of wind. These shortcomings severely limit the maneuverability of such a craft and requires the application of considerably unconventional sailing techniques.